<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a{
            width: 200px;
            height: 100px;
            background: #000;
            /* text-align: center; */
            line-height: 100px;
            color: white;
            /* font-size: 32px; */
            margin: 0 auto;
            text-indent: 35px;
        }
        .as{
            width: 0;
            height: 0;
            display: inline-block;
            border: 5px solid transparent;
            border-top: 5px solid red;
            position: relative;
            top: 2.5px;
            
        }
        .a:hover .as{
            width: 0;
            height: 0;
            display: inline-block;
            border: 5px solid transparent;
            border-bottom: 5px solid red;
            position: relative;
            top: -2.5px;
        }
    </style>
</head>
<body>
    <div class="a">导航
        <span class="as"></span>
    </div>
</body>
</html>